Conventional sputtering devices include a vacuum chamber enclosing a target electrode of selected material and a substrate onto which the material is to be deposited. Air within the chamber is evacuated to a low pressure and is partially replaced by an ionizable gas, such as argon. A power supply applies a negative potential to the target electrode. Gas ions strike the target, causing an emission of atoms from the target into a plasma from which the target material is deposited on the substrate.
In certain applications, such as the manufacture of magnetic recording heads, thin films of magnetic material need to be applied with a predetermined magnetic orientation. The thin films are deposited by sputtering apparatus in a predetermined orientation by exposing the substrate to a uniform magnetic field (i.e., equal magnitude field lines extending in a common direction). Ordinarily, permanent magnets placed in the vicinity of the substrate are used to generate the magnetic field.
For example, most commercial implementations locate a pair of permanent magnets on opposite sides of the substrate to generate the required field across the substrate. However, only a small portion of the magnetic field between the electromagnets exhibits the necessary uniformity, and this limits the area of the substrate over which the target material can be deposited with the required magnetic orientation.
U.S. Pat. No. 5,026,470 to Bonyhard et al. discloses an alternatively designed sputtering apparatus having a polygon-shaped electromagnet located beneath the substrate. A spiral coil having a plurality of sides forming a polygon is embedded in a pallet assembly for producing a plurality of uniform magnetic fields that extend perpendicular to each of the sides. A plurality of substrates is positioned on the pallet assembly with respective edges aligned with one of the sides. However, this apparatus is very large, difficult to manufacture, and inefficient to operate with smaller lot sizes.
After coating and subsequent processing, the substrates are cut into smaller units that are used for such purposes as heads for disk drives. An industry trend now requires larger substrates (e.g., 15.25 centimeters squared) to be coated with more accurately aligned magnetic domains (e.g., within one degree alignment). The increased accuracy provides improved yield from the substrate, and the increased area of the substrate permits more heads to be manufactured simultaneously.
The larger substrates are difficult to coat with the present designs of sputtering apparatus. For example, the permanent magnets located at either side of the substrate must be spaced at large distances to produce the required uniformity. However, the added spacing requires impractically large magnets be used to produce the required field strength. Similarly, interferences between magnetic fields on each side of the pallet assembly of Bonyhard et al. can significantly limit the size of substrates that can be coated with accurately aligned magnetic material.
Although a uniform magnetic field is required in the vicinity of the substrate for magnetically orienting particles of the target material as they are deposited on the substrate, the same magnetic field in the vicinity of the target can cause uneven erosion of the target and variations in the thickness of deposited target material across the surface of the substrate. The magnetic field reacts with an electric field in the vicinity of the target causing emitted electrons to drift across the target and to increase local ionization and accompanying bombardment of one end of the target.
The sputtering device of Bonyhard et al. provides for rotating the pallet assembly to provide a more uniform coating of the target material on the substrates. However, this adds to the size and complexity of the device and requires the use of uniformity shields which cut off part of the sputtered flux in order to produce a uniform film deposition.